1. Field of the Invention
The present invention relates to a motor that is used in an electronic device or the like.
2. Description of the Related Art
In the related art, a disk spindle motor having the structure shown in FIG. 12, an axial flow fan motor having the structure shown in FIG. 13, and the like have been known as motors used in electronic devices or the like. Both the motors shown in FIGS. 12 and 13 have the following structure. That is, a radial bearing 133 supports a rotating shaft so that the rotating shaft can be rotated, or the radial bearing supports a rotor so that the rotor can be rotated with respect to the shaft. A slide bearing is used as the radial bearing 133. The shaft 131 maintains the rotation of the rotor 111 with respect to the stator 112.
The disk spindle motor 101A shown in FIG. 12 should constantly maintain the position of a turntable 102 in order to stably record and reproduce information signals on/from an optical disk. Further, the axial flow fan motor 101B shown in FIG. 13 should support the weight of an impeller 119 in every direction and attitude, and should stably rotate the impeller 119 without a thrust force during the rotation of the impeller 119. Consequently, each of the motors 101A and 101B using the radial bearing 133 that is a slide bearing and the thrust bearing 134 should generate a pressing force (hereinafter, referred to as “thrust attraction”) that presses the shaft 131 against the thrust bearing 134. A method of generating the pressing force that presses the shaft 131 against the thrust bearing 134 will be described hereinafter with reference to FIG. 14. Meanwhile, FIG. 14 is an enlarged view showing the positional relationship between a magnet 120 and a core 115, which generate thrust attraction, of each of the motors 101A and 101B shown in FIGS. 12 and 13.
As shown in FIG. 14, a motor generating thrust attraction has the structure in which a magnetic center C01 of the core 115 in a thrust direction is deviated and offset from a magnetic center C02 of the magnet 120 in the thrust direction by a predetermined distance L in the thrust direction. In this motor, lines of magnetic force generated from the magnet 120 proceed to the core 115. However, since the magnetic centers are offset from each other by the distance L, the states of the lines of magnetic force are different from each other at one side (upper side) and the other side (lower side) of the core 115 in the thrust direction. In this case, the lines of magnetic force are formed along the shortest distance so that magnetic resistance is minimized.
When the distance L representing the deviation (offset distance) between the magnetic centers is changed, if L is 0, a state is stable. When the offset distance is changed from the stable state, the lines of magnetic force are different from each other at the upper and lower sides of the core. Accordingly, repulsive thrust is generated in the thrust direction due to the difference of the lines of magnetic force, and the magnetic force is increased at the upper side of the core. As a result, thrust attraction Fs that is a force attracting the shaft 131 to the thrust bearing 134 is generated. Meanwhile, thrust attraction generated due to the offset between the center of the core 115 and the center of the magnet 120 is used in various motor other than the disk spindle motor 101A and the axial flow fan motor 101B.
However, although the above-mentioned motor generating thrust attraction is inexpensive and excellent in terms of the method of attracting the shaft in the thrust direction, there is a concern that noise and vibration are generated as described below. The principle of the generation of noise and vibration will be described below with reference to FIGS. 15A to 15D.
FIGS. 15A to 15D and 16A to 16D are views illustrating the relationship between the core 115 around which a coil 114 are wound (hereinafter, referred to as a “core coil”) and the magnet 120, as seen from the outside in FIG. 14, that is, side views of the core coil 114 and 115 and the magnet 120 as seen from the outside to the inside in a radial direction and cross-sectional views showing the positional relationship between the core coil 114 and 115 and the magnet 120 in the thrust direction. Meanwhile, FIGS. 15A to 15C are side views showing that thrust attraction is generated due to the offset corresponding to a distance L, and FIG. 15D is a cross-sectional view showing each of the states of FIGS. 15A to 15C and an offset state corresponding to a distance L. Further, FIGS. 16A to 16C are side views showing that offset does not occur due to L=0 and thrust attraction is not generated, and FIG. 16D is a cross-sectional view showing each of the states of FIGS. 16A to 16C and a distance L is 0.
FIGS. 15A, 15B, and 15C are views showing that the relationship between the core 115 and the magnet 120 facing the core is changed due to the rotation of the rotor 111. That is, FIG. 15A shows that the core 115 faces a portion of the magnet 120 magnetized to an N pole. FIG. 15B shows that the rotor 111 is slightly rotated from the state of FIG. 15A, that is, the core 115 faces boundary portions of the magnet 120 magnetized to an N pole and an S pole. FIG. 15C shows that the rotor 111 is further rotated slightly from the state of FIG. 15B, that is, the core faces a portion of the magnet magnetized to an S pole of the boundary portions of the magnet 120 magnetized to an N pole and an S pole.
In any state among the states of FIGS. 15A, 15B, and 15C, the core and the magnet are positioned in a relationship where the magnetic centers of the core 115 and the magnet 120 are offset from each other in the thrust direction (hereinafter, this state is referred to as “magnetic center offset”) as shown in FIG. 15D. Accordingly, the shaft 131 is attracted to the thrust bearing 134 due to thrust attraction Fs that is caused by the magnetic center offset.
However, the magnetic center offset between the core 115 and the magnet 120 has an effect on the coil 114 wound around the core 115. That is, relatively high density magnetic flux M01 contributes from the magnet 120 to one side coil 114a corresponding to one side, where the magnetic center C02 of the magnet 120 is deviated and offset from the magnetic center C01 of the core 115, of both sides of the core 115 in the thrust direction, among the coil 114 wound around the core 115. Relatively low density magnetic flux M02, which is generated from the magnet 120, contributes to the other side coil 114b. In other words, the magnetic flux, which is generated from the magnet 120 and does not contribute to torque rotating the rotor 111, is orthogonal to the coil 114.
For example, due to the effect of the magnetic flux, in the state shown in FIG. 15A, constant thrust attraction Fs is generated as described above, and an unnecessary force F01 is generated in one side coil 114a, which is provided on one side of the core 115 in the thrust direction, among the coil 114 wound around the core 115 in the opposite direction X2 opposite to the direction X1 of the thrust attraction Fs. Meanwhile, a force F02 is also generated in the other side coil 114b, which is provided on the other side of the core 115 in the thrust direction, among the coil 114 wound around the core 115 in the same direction X1 as the direction of the thrust attraction Fs. However, since the magnetic flux M02 having an effect as described above has density lower than the magnetic flux M01, it may not be possible to offset the unnecessary force F01 generated in the opposite direction X2.
In this case, assuming that current flows through the coil 114 in the direction shown in FIG. 15A, if the so-called Fleming's left-hand rule is applied in a state where the N pole is dominant as shown in FIG. 15A, it is apparent that the generated unnecessary forces F01 and F02 are generated in the thrust direction and generated as described above in the opposite direction X2 to the direction of the thrust attraction Fs on one side of the core in the thrust direction. Further, it is apparent that the unnecessary forces are generated in the same direction X1 as the direction of the thrust attraction Fs on the other side of the core in the thrust direction. Meanwhile, the motor is similar to a common motor in the related art in that the phases of the N and S poles of the magnet 120 are detected by a Hall element 119 and the value of the current flowing through the coil 114 is changed in time. Meanwhile, relatively small current is supplied to the coil in the state of FIG. 15A, maximum current is supplied to the coil in the state of FIG. 15B to be described below, and relatively medium current is supplied to the coil in the state of FIG. 15C. Likewise, small current is supplied to the coil in the state of FIG. 16A, maximum current is supplied to the coil in the state of FIG. 16B, and medium current is supplied to the coil in the state of FIG. 16C.
Further, the state shown in FIG. 15B is similar to the above-mentioned state shown in FIG. 15A in that the magnetic flux has high and low density at one side and the other side coils 114a and 114b. However, since the facing portion of the magnet 120 is the boundary portion between the N pole and the S pole, the unnecessary forces are offset by the magnetic flux generated from the N pole and the S pole at any one of one side and the other side coils 114a and 114b. Therefore, unnecessary forces are not generated, and only constant thrust attraction Fs is generated as described above.
Further, the state shown in FIG. 15C is reversed to the state described with reference to FIG. 15A, and constant thrust attraction Fs is generated as described above. In addition, an unnecessary force F03 is generated in one side coil 114a, which is provided on one side of the core 115 in the thrust direction, among the coil 114 wound around the core 115 in the same direction X1 as the direction of the thrust attraction Fs. Meanwhile, a force F04 is generated in the other side coil 114b, which is provided on the other side of the core 115 in the thrust direction, among the coil 114 wound around the core 115 in the opposite direction X2 to the direction of the thrust attraction Fs. However, since the magnetic flux M04 having an effect as described above has density lower than the magnetic flux corresponding to one side coil, it may not be possible to offset the unnecessary force F03 generated in the same direction.
In this case, if current flows through the coil 114 in the direction shown in FIG. 15C, the state is changed into the state where the S pole is dominant as shown in FIG. 15C, and the so-called Fleming's left-hand rule is applied, it is apparent that the generated unnecessary forces F03 and F04 are generated in the thrust direction and generated as described above in the same direction X1 as the direction of the thrust attraction Fs on one side of the core in the thrust direction. Further, it is apparent that the unnecessary forces are generated in the opposite direction X2 to the direction of the thrust attraction Fs on the other side of the core in the thrust direction.
As described above, the unnecessary forces are repeatedly generated while the directions of the unnecessary forces are changed in the states of FIGS. 15A to 15C due to the rotation of the rotor 111. Accordingly, while being changed in the direction X1 or X2 that is toward the upper or lower side in the thrust direction, a small force is applied to the rotor 111 or the rotor minutely moves due to the small force. For this reason, there has been a concern that noise and vibration are generated.
In contrast to the state where thrust attraction is generated as shown in FIGS. 15A to 15C, in the state where offset does not occur due to L=0 as shown in FIGS. 16A to 16C and thrust attraction is not generated, the rotor 111 does not minutely move unlike in FIGS. 15A to 15C and there is no concern that undesirable vibration and noise are generated.
That is, in any state of the states of FIGS. 16A to 16C, the core and the magnet are positioned in a relationship where the magnetic centers C01 and C02 of the core 115 and the magnet 120 coincide with each other as shown in FIG. 16D.
Since the magnetic centers of the core 115 and the magnet 120 coincide with each other, there is no effect on the coil 114 wound around the core 115, that is, substantially the same density magnetic flux M03 and M04 contribute to both one side and the other side coils 114a and 114b corresponding to both sides of the core 115 in the thrust direction among the coil 114 wound around the core 115.
For example, in the state shown in FIG. 16A, an unnecessary force F05 is generated in one side coil 114a among the coil 114 wound around the core 115 in one direction X2 (toward the upper side in FIG. 16A). However, at the same time, an unnecessary force F06 is generated in the other side coil 114b among the coil 114 wound around the core 115 in the opposite direction X1 (toward the lower side in FIG. 16A) to the direction of the unnecessary force F05 that is generated in one side coil 114a. Accordingly, substantially the same density of magnetic flux M03 and M04 have an effect on one side and the other side coils 114a and 114b, so that the forces are offset to each other.
Further, in the state shown in FIG. 16B, like the case of FIG. 15B, the unnecessary forces are offset by the magnetic flux generated from the N pole and the S pole at any one of one side and the other side coils 114a and 114b. Therefore, the unnecessary forces are not generated.
In addition, in the state shown in FIG. 16C, like the case of FIG. 16A, an unnecessary force F07 is generated in one side coil 114a among the coil 114 wound around the core 115 in the other direction X1. However, at the same time, an unnecessary force F08 is generated in the other side coil 114b among the coil 114 wound around the core 115 in the opposite direction X2 to the direction of the unnecessary force F07 that is generated in one side coil 114a. Accordingly, substantially the same density of magnetic flux M03 and M04 have an effect on one side and the other side coils 114a and 114b, so that the forces are offset to each other.
When the magnetic center offset between the core 115 and the magnet 120 that has been described with reference to FIGS. 16A to 16C does not occur, the minute movement of the rotor 111 occurring in the thrust direction in the cases of FIGS. 15A to 15C or undesirable vibration and noise caused by the minute movement are not generated.
However, when the magnetic center offset between the core 115 and the magnet 120 does not occur as shown in FIGS. 16A to 16C, the thrust attraction necessary to function as a motor should be generated by the separate structure.
In the related art, there has been a motor that obtains thrust attraction by using a dedicated thrust-attraction magnet like a motor disclosed in JP-A-11-252878. Specifically, there have been a disk spindle motor 141A and an axial flow fan motor 141B shown in FIGS. 17 and 18. The structure of a thrust-attraction magnet 142 used in FIGS. 17 and 18 will be described in detail below with reference to FIG. 19. Meanwhile, FIG. 19 is an enlarged view of a portion, on which the thrust-attraction magnet 142 is mounted, of each of the motors shown in FIGS. 17 and 18. In FIGS. 17 to 19 and the following description, the same components as the components of the motors shown in FIGS. 12 and 13 are indicated by the same reference numerals and the detailed description thereof will be omitted.
Each of the motors 141A and 141B is provided with a thrust-attraction magnet 142, which includes a back yoke 143, on a stator 112 as shown in FIG. 19. The thrust-attraction magnet 142 attracts a rotor yoke 111a, and generates thrust attraction that attracts a rotor 111 and a shaft 131, which is integrally fixed to the rotor, to a thrust bearing 134. Each of the motors shown in FIGS. 17 and 18 generates thrust attraction by using the above-mentioned thrust-attraction magnet 142, and is to avoid unnecessary vibration and noise. Meanwhile, although each of the motors 141A and 141B shown in FIGS. 17 and 18 has been provided with the magnet 142 on the stator 112, the magnet may be provided on the rotor 111 and an attraction yoke that attracts a magnetic body may be provided on the stator 112. Meanwhile, a neodymium magnet, which has relatively large energy product, has been used as the thrust-attraction magnet in order to stabilize attraction and prevent variation.
However, according to each of the motors 141A and 141B including the thrust-attraction magnet 142, manufacturing cost is increased due to the addition of the structure, the limitation on a mounting space is increased, and an air gap ga between the magnet and the attraction yoke should be maintained. For this reason, if the structure against vibration and noise is added, there is a problem in that cost is increased. Further, it is difficult to simplify the structure, to reduce manufacturing cost, and to suppress unnecessary vibration and noise, at the same time.